RxJS Interview Questions and Answers for freshers

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RxJS Interview Questions for Freshers

  1. What is RxJS?

    • Answer: RxJS (Reactive Extensions for JavaScript) is a library for reactive programming using Observables, to make it easier to compose asynchronous or callback-based code.

  2. What are Observables?

    • Answer: Observables are objects that represent a stream of data over time. They can emit multiple values, an error, or a completion notification.

  3. What are Observers?

    • Answer: Observers are functions that subscribe to an Observable and react to the values, errors, or completion notifications it emits.

  4. Explain the difference between Observables and Promises.

    • Answer: Promises represent a single future value, while Observables represent a stream of multiple values over time. Promises resolve once; Observables can emit multiple values before completing.

  5. What is the `subscribe()` method?

    • Answer: `subscribe()` is the method used to attach an Observer to an Observable. It specifies how to handle emitted values, errors, and completion.

  6. What are operators in RxJS?

    • Answer: Operators are functions that transform or manipulate Observables. They allow you to create complex asynchronous workflows.

  7. Name some common RxJS operators.

    • Answer: `map`, `filter`, `reduce`, `merge`, `concat`, `switchMap`, `debounceTime`, `take`, `retry` are some common operators.

  8. Explain the `map` operator.

    • Answer: The `map` operator transforms each emitted value from an Observable using a provided function and emits the transformed values.

  9. Explain the `filter` operator.

    • Answer: The `filter` operator filters the emitted values from an Observable based on a provided predicate function, emitting only values that satisfy the condition.

  10. Explain the `reduce` operator.

    • Answer: The `reduce` operator accumulates all emitted values from an Observable into a single value using a provided reducer function.

  11. Explain the `merge` operator.

    • Answer: The `merge` operator combines multiple Observables into a single Observable, emitting values from all source Observables as they arrive.

  12. Explain the `concat` operator.

    • Answer: The `concat` operator concatenates multiple Observables into a single Observable, emitting values from each Observable sequentially.

  13. Explain the `switchMap` operator.

    • Answer: `switchMap` projects each emitted value from the source Observable into an inner Observable. It cancels previous inner Observables when a new value is emitted.

  14. Explain the `debounceTime` operator.

    • Answer: `debounceTime` suppresses emissions from an Observable for a specified time period. Only the last emitted value within the time period is emitted.

  15. Explain the `take` operator.

    • Answer: `take` emits only the first `n` number of emitted values from an Observable and then completes.

  16. Explain the `retry` operator.

    • Answer: `retry` resubscribes to an Observable after it encounters an error, up to a specified number of times.

  17. What is a Subject in RxJS?

    • Answer: A Subject is a special type of Observable that allows values to be both pushed into and pulled from. It acts as a bridge between different parts of your application.

  18. What are the different types of Subjects?

    • Answer: `Subject`, `BehaviorSubject`, `ReplaySubject`, and `AsyncSubject` are the common types.

  19. Explain `BehaviorSubject`.

    • Answer: `BehaviorSubject` emits the last emitted value to new subscribers, even if the subscription happens after the value was emitted.

  20. Explain `ReplaySubject`.

    • Answer: `ReplaySubject` replays all emitted values to new subscribers, up to a specified buffer size.

  21. Explain `AsyncSubject`.

    • Answer: `AsyncSubject` emits only the last emitted value when the source Observable completes.

  22. What is the difference between `hot` and `cold` Observables?

    • Answer: Hot Observables emit values independently of subscribers. Cold Observables emit values only when subscribed to.

  23. How do you handle errors in RxJS?

    • Answer: Errors are handled using the `catchError` operator or the error handler function in the `subscribe` method.

  24. What is the `catchError` operator?

    • Answer: The `catchError` operator handles errors emitted by an Observable, allowing you to recover from errors or handle them gracefully.

  25. How do you unsubscribe from an Observable?

    • Answer: Unsubscribe using the `unsubscribe()` method returned by the `subscribe()` method. This prevents memory leaks.

  26. What is the `finalize` operator?

    • Answer: `finalize` performs an action (like cleanup) whether the Observable completes successfully or due to an error.

  27. What are schedulers in RxJS?

    • Answer: Schedulers control when Observables emit values. They allow you to control the timing and execution context of asynchronous operations.

  28. Name some common RxJS schedulers.

    • Answer: `async`, `queue`, `animationFrame`, `immediate` are some examples.

  29. What is the `async` scheduler?

    • Answer: The `async` scheduler uses the browser's `setTimeout` or Node.js's `setImmediate` to schedule tasks.

  30. What is the `queue` scheduler?

    • Answer: The `queue` scheduler uses a queue to execute tasks in the order they are added, ensuring sequential execution.

  31. What is the `animationFrame` scheduler?

    • Answer: The `animationFrame` scheduler schedules tasks to run before the next browser repaint, optimizing for UI updates.

  32. What is the `immediate` scheduler?

    • Answer: The `immediate` scheduler executes tasks synchronously, useful for simple operations where no delay is needed.

  33. How can you test RxJS code?

    • Answer: Use tools like Jest and libraries that provide testing utilities for Observables, such as `rxjs/testing`.

  34. How do you create an Observable from an array?

    • Answer: Use the `from` or `of` operator. `from` emits each element individually; `of` emits the array as a single value.

  35. How do you create an Observable from an event?

    • Answer: Use the `fromEvent` operator, providing the target element and the event name.

  36. How do you create an Observable that emits values periodically?

    • Answer: Use the `interval` operator.

  37. How do you create an Observable that emits a single value after a delay?

    • Answer: Use the `timer` operator.

  38. What is the `shareReplay` operator?

    • Answer: `shareReplay` allows multiple subscribers to share the same Observable instance, replaying values to new subscribers.

  39. What is the difference between `share` and `shareReplay`?

    • Answer: `share` multicasts the Observable, but doesn't replay values. `shareReplay` replays emitted values to new subscribers.

  40. How do you handle HTTP requests with RxJS?

    • Answer: Use libraries like `rxjs/ajax` or combine RxJS with libraries like `axios` to make HTTP calls and handle the responses as Observables.

  41. What is the purpose of the `distinctUntilChanged` operator?

    • Answer: It suppresses consecutive emissions of identical values.

  42. What is the purpose of the `distinctUntilKeyChanged` operator?

    • Answer: It suppresses consecutive emissions of objects that have the same value for a specific key.

  43. Explain the `auditTime` operator.

    • Answer: It only emits the last value emitted by the source observable within a specified time window.

  44. Explain the `audit` operator.

    • Answer: It only emits values from the source observable after a specified duration since the last emission has passed.

  45. What is the `buffer` operator used for?

    • Answer: It groups emitted values into buffers based on different criteria (e.g., time, count).

  46. What is the `bufferCount` operator?

    • Answer: It emits buffers containing a specified number of values.

  47. What is the `bufferTime` operator?

    • Answer: It emits buffers at specified time intervals.

  48. What is the `bufferWhen` operator?

    • Answer: It emits buffers based on a specified closing Observable.

  49. What is the `delay` operator?

    • Answer: It delays the emission of values from an Observable by a specified duration.

  50. What is the `delayWhen` operator?

    • Answer: It delays the emission of values based on another Observable.

  51. What is the `expand` operator?

    • Answer: It recursively projects each emitted value into an Observable and concatenates the results.

  52. What is the `exhaustMap` operator?

    • Answer: It ignores new emissions from the source Observable until the previous inner Observable completes.

  53. What is the `first` operator?

    • Answer: It emits only the first value from the source Observable.

  54. What is the `groupBy` operator?

    • Answer: It groups the emitted values into different Observables based on a key selector function.

  55. What is the `ignoreElements` operator?

    • Answer: It ignores all emitted values and only emits a completion notification when the source Observable completes.

  56. What is the `last` operator?

    • Answer: It emits only the last value emitted by the source Observable.

  57. What is the `mapTo` operator?

    • Answer: It replaces each emitted value with a specified constant value.

  58. What is the `materialize` operator?

    • Answer: It transforms the Observable's emissions into Notification objects.

  59. What is the `partition` operator?

    • Answer: It splits the Observable into two Observables based on a predicate function.

  60. What is the `pluck` operator?

    • Answer: It extracts a property from emitted objects.

  61. What is the `publish` operator?

    • Answer: It creates a connectable Observable that can be connected to multiple subscribers.

  62. What is the `publishReplay` operator?

    • Answer: It's a connectable Observable that replays values to new subscribers.

  63. What is the `race` operator?

    • Answer: It emits values from the first Observable among multiple Observables that emits a value.

  64. What is the `sample` operator?

    • Answer: It emits the last value from the source Observable when a specified notifier Observable emits a value.

  65. What is the `sampleTime` operator?

    • Answer: It emits the last value emitted by the source Observable within a specified time interval.

  66. What is the `scan` operator?

    • Answer: It accumulates values over time, similar to `reduce`, but emits each intermediate result.

  67. What is the `skip` operator?

    • Answer: It ignores the first `n` emitted values.

  68. What is the `skipUntil` operator?

    • Answer: It ignores emissions until a specified notifier Observable emits a value.

  69. What is the `skipWhile` operator?

    • Answer: It ignores emissions while a specified predicate function returns true.

  70. What is the `startWith` operator?

    • Answer: It prepends a specified value or values to the beginning of the emitted sequence.

  71. What is the `takeUntil` operator?

    • Answer: It emits values until a specified notifier Observable emits a value.

  72. What is the `takeWhile` operator?

    • Answer: It emits values as long as a specified predicate function returns true.

  73. What is the `throttle` operator?

    • Answer: It suppresses frequent emissions, emitting only the last value within a specified duration.

  74. What is the `throttleTime` operator?

    • Answer: It suppresses emissions for a specified time period, emitting only the last value within that time.

  75. What is the `timeout` operator?

    • Answer: It throws an error if the Observable doesn't emit a value within a specified timeout period.

  76. What is the `timeoutWith` operator?

    • Answer: It switches to another Observable if the source Observable doesn't emit within a timeout period.

  77. What is the `toArray` operator?

    • Answer: It collects all emitted values into an array and emits the array as a single value.

  78. What is the `withLatestFrom` operator?

    • Answer: It emits the last value from the source Observable when another Observable emits a value.

  79. What is the `zip` operator?

    • Answer: It combines multiple Observables, emitting values as tuples from corresponding emissions.

  80. Explain the concept of backpressure in RxJS.

    • Answer: Backpressure refers to the scenario where a slow subscriber cannot keep up with a fast Observable. Strategies to handle this include buffering, dropping, or throttling emissions.

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